It is general that luggage to be loaded onto an aircraft is previously inspected at an airport to secure the safety operation of the aircraft. As a luggage inspection system, it is general to use a transmission X-ray inspection apparatus using transmission of X-rays or a Compton scattered X-ray inspection apparatus using Compton scattering of X-rays. By the X-ray inspection apparatus using transmitted X-rays or Compton scattered X-rays, transmitted X-rays or Compton scattered X-rays are led to an X-ray detector. The detected X-rays are converted into visible light by a phosphor. The intensity of visible light is detected by a photomultiplier. The inside of the luggage is imaged according to the intensity of visible light to inspect the luggage.
To improve luggage inspection accuracy, it is necessary to obtain a clearer image. For that purpose, it is required to input a sufficient intensity of visible light to the photomultiplier. The intensity of visible light can be enhanced according to the intensity of X-rays irradiated to the luggage or the like. But, when the intensity of X-rays irradiated is enhanced, the X-ray inspection apparatus which is installed in a public place such as an airport luggage inspection system becomes large in size, and a risk increases. Therefore, there is a demand for a phosphor having a high efficiency of converting X-rays into visible light. When the phosphor excelling in efficiency of converting to the visible light is used, high luminance visible light can be obtained without enhancing the irradiation intensity of X-rays, and it is possible to input the visible light having sufficient intensity to the photomultiplier as a result.
As the phosphor used for the X-ray inspection apparatus, there are known a rare earth oxysulfide phosphor having a composition represented by A2O2S:D (A is at least one element selected from Gd, La and Y, and D is at least one element selected from Tb and Pr or a mixture of this element and at least one element selected from Ce and Yb), and a barium halide phosphor having a composition represented by BaFX:E (X is at least one element selected from Cl and Br, and E is Eu or a mixture of Eu and at least one element selected from Ce and Yb). These phosphors are effective as phosphors for the X-ray inspection apparatus because they are excellent in efficiency of converting the transmitted X-rays or the Compton scattered X-rays into visible light.
Since the inspected luggage diversifies in these days, there are demands that complicated shapes are determined more accurately. When the intensity of the irradiated X rays is enhanced to improve the inspection sensitivity by enhancing the inspection image, the X-ray inspection apparatus is made to have a large size as described above, and a risk increases. Therefore, there are strong demands for an X-ray detector which provides satisfactory detection sensitivity even when the irradiation X-rays of relatively low intensity are used. In addition, there are strong demands for an X-ray inspection apparatus which is small, has less risk and can obtain a clear inspection image by using the above X-ray detector.